In semiconductor integrated circuits, there has conventionally been an SOI (Silicon On Insulator) structure in which elements are separated by an insulating layer. A bonding method is known as one of manufacturing methods for realizing the SOI structure. One type of this method is reported in Documents 1 and 2 noted below.
Document 1: Appl. Phys. lett. Vol. 64, No. 16, (T. Yonehara, et al) pp. 2108-2110 (1994)
Document 2: IEICE Trans. Electronics, E80-C, (K. Sakaguchi, et al) pp. 378-387 (1997)
The method reported in Documents 1 and 2 noted above is called an ELTRAN (Epitaxial Layer Transfer) method.
Further, in Document 3 (Japanese Unexamined Patent Application Publication (JP-A) No. 2002-270801), there is proposed a method for uniformly controlling the impurity concentration of a surface single-crystal silicon layer (SOI layer) being an epitaxially grown layer.
Referring to FIG. 1, the conventional semiconductor substrate manufacturing method described in Document 3 noted above will be schematically explained. First, as shown in FIG. 1 at (a), a first semiconductor substrate 201 is prepared. As the first semiconductor substrate 201, use is made of, for example, a boron-doped p-type high-concentration substrate having a boron concentration of about 5×1018 cm−3 and a resistivity of about 0.01 to 0.02 Ωcm. Subsequently, the surface of the first semiconductor substrate 201 is formed porous in a solution containing HF (Hydrogen Fluoride) or the like according to an anodizing method or the like, thereby forming a porous layer 202 as shown in FIG. 1 at (b).
Then, as shown in FIG. 1 at (c), a non-porous single-crystal silicon layer 203 is formed on the porous layer 202, wherein an oxide film or a nitride film is formed through a heat treatment at 300° C. to 500° C. as a pore-wall protective film for the porous layer 202 and the protective film is removed only at a surface portion of the porous layer 202.
Further, as shown in FIG. 1 at (d), a back seal film 204 is formed on the back side of the first semiconductor substrate 201. Then, as shown in FIG. 1 at (e), an epitaxially grown layer 205 is formed on the non-porous single-crystal silicon layer 203.
Subsequently, as shown in FIG. 1 at (f), a second semiconductor substrate 206 is bonded to the epitaxially grown layer 205 through an insulator layer 207 of silicon oxide or the like.
Then, the porous layer 202 is exposed by removing the back seal film 204 and the first semiconductor substrate 201 and separating the first semiconductor substrate 201 side and the second semiconductor substrate 206 side from each other at the porous layer 202 (FIG. 1 at (g)). In FIG. 1 at (g), the front and back sides in FIG. 1 at (f) are shown reversely.
Thereafter, the remaining unnecessary porous layer 202 is removed by etching, polishing, or the like, thereby exposing the non-porous single-crystal silicon layer 203 and the second semiconductor substrate 206 on both the front and back sides (FIG. 1 at (h)). Then, the surface of the non-porous single-crystal silicon layer 203 is smoothed depending on necessity. In this manner, there is manufactured an SOI wafer composed of the second semiconductor substrate 206, the insulating layer 207, and the epitaxially grown layer 205 (FIG. 1 at (i)).
On the other hand, in the method described in Document 3, when a heat treatment at 1000° C. or more is applied to the porous layer 202, a structural change occurs. In view of this, Document 3 describes that a protective film of silicon oxide, silicon nitride, or the like is preferably formed on the pore walls in a thickness of about 2 to 10 nm as the pore-wall protective film for the porous layer 202.
Document 3 describes that the pore-wall protective film is formed on the porous layer 202 through the heat treatment in an oxygen atmosphere at 300° C. to 500° C. However, when the silicon oxide film is formed in the oxygen atmosphere at the low temperature such as 300° C. to 500° C., the oxidation rate of silicon is extremely slow so that the manufacturing efficiency is quite low for forming the thickness of 2 to 10 nm.
Further, the properties of the oxide film formed in the oxygen atmosphere at the low temperature such as 300° C. to 500° C. are poor as compared with those of an oxide film formed at 1000° C. and thus its function as the protective film is low.
It is therefore an object of this invention that when forming a protective film, the manufacturing efficiency is improved at the low temperature such as 300° C. to 500° C. and, further, improvement in film properties can be realized.